Combustion engines on the basis of Otto Motors are usually operated with fuel consisting of hydrocarbons of fossil fuels based on refined petroleum. This fuel is increasingly admixed in different mixture rates with alcohol generating from renewable raw materials (plants), for example ethanol or methanol. In the US and Europe a mixture of 75-85% ethanol and 15-25% gasoline is applied under the trade name E85. The combustion engines are so construed that they can be operated with pure gasoline as well as with mixtures right up to E85; this is called “flex-fuel-operation”. For an economical operation with a low emission of hazardous substances concurrently with a high engine performance the running parameters in the flex-fuel-operation have to be adjusted to the currently present fuel mixture. By way of example a stoichiometric air-fuel ratio is present at 14.7 parts of volume of air per part of gasoline, but when using ethanol one air part of 9 parts of volume has to be adjusted.
The momentary fuel composition before the fuel injection timing and the momentary exhaust gas composition, the oxygen partial pressure in the exhaust gas, are determined by the interaction of sensors and then transferred to the control electronics of the combustion engine. On the basis of this sensor data the combustion of the combustion engine is optimized, especially by adjusting the advantageous air-fuel ratio.
A first sensor can be a fuel sensor, also called “fuel composition sensor”. Fuel sensors use the various features of alcohol and gasoline for determining the fuel composition. Ethanol for example is a protic solvent, which contains hydrogen ions and a high dielectric constant that depends on the water content. Gasoline on the other hand is an aprotic solvent with a low dielectric constant. Based on this there are fuel sensors, which determine the fuel composition with the aid of the dielectric features of the fuel mixture. Other fuel sensors use various optic features of fuel, for example the different indices of refraction. Both sensor types are expensive and susceptible in its functioning.
The second sensor for example is a “downstream oxygen sensor”, which determines the oxygen content in the exhaust gas behind a catalytic converter. Thereby it can be a stoichiometric or a wide-band lambda probe. Alternatively also a gas-selective exhaust gas probe, for example a nitric oxide probe, can be provided.
The fuel sensor determines the R—OH-parts in the fuel, whereby R describes hydrogen or different hydrocarbon-remains. On the basis of the sensor signals the control of the fuel pre-heating and of the temperature during the injection or the control of the injection- and ignition-moment as well as the compression of the fuel take place.
The lambda probe determines the oxygen partial pressure in the exhaust gas of the combustion engine, the regulating position of the combustion engine regarding a rich/lean setting as well as the regularization of the air-fuel ratio over the air volume and d the injection quantity.
Together the sensors take over the combustion regularization in the combustion engine. Thereby the information of the sensors is complementary. Since the oxygen supply of the fuel mixture or rather the air-fuel ratio and the regulating position during a preset fuel composition directly relate to each other, the system is partially over determined by the information that has been delivered by the sensors.
Due to DE 411 74 40 C2 a procedure is known for the adaptive setting of a fuel-air mixture for considering the fuel features during the operation of a combustion engine, which provides a lambda regulator that displays a regulating factor RF, and which provides an adaptation integrator, which displays an adaptation factor AF with a variable adaptation speed that influences the setting of the fuel-air mixture besides the regulating factor RF. Thereby it is provided that it is checked whether the lambda-regularization-deviance-amplitude exceeds a first threshold value, and, if this is the case, whether the adaptation speed is set at an increased level so long until a preset condition is achieved, after which a low adaptation speed is switched back on.
The procedure allows combustion engines, which can be operated with different fuels, to operate failure-free. Thus the injection time for example has to be extended by more than 20% during a change from a fuel gasoline to a fuel mixture of 85% ethanol and 15% gasoline, in order to achieve the same lambda values in the exhaust gas. According to the procedure described in the script DE 411 74 40 C2 a corresponding adaptation interference is undertaken therefore. In the suggested procedure the adaptation speed is significantly increased during a recognized fuel change, because a very intense correction of the injection times and therefore of the adaptation interference has to be undertaken during a fuel change, compared to the correction of aging influences or production influences.
On the basis of the set adaptation value the fuel mixture ratio can be determined. Despite the increased adaptation speed the procedure requires a sufficient long settling time. If a significant change of the fuel mixture ratio is caused by refueling, this can lead to startup problems and to combustion misses, which leads to increased exhaust gas emissions. Here the described fuel sensor can undertake a quick determination of the exhaust gas composition.
It is the inventions task to provide a procedure, which allows a reliable and economic detection of the composition of a fuel mixture consisting of at least two fuels, whereby fuel mixtures of different compositions require different air-fuel ratios to achieve a stoichiometric combustion.